(a) Field of the Invention
The present invention relates to a resin used in a toner for use in-the electrophotography and more specifically to a degradable binder resin for electrophotographic toners.
(b) Description of the Prior Art
The electrophotography technique comprises the steps of forming, through a variety of means, electrostatic latent images on a light-sensitive material while making use of a photoconductive substance, then developing the latent images with a toner, optionally transferring the images on an image-support such as paper and thereafter fixing the toner images thus formed by passing through, for instance, heated rolls to thus give desired images. There have been investigated a variety of substances for use as resins for preparing toners. Among these, polyester resins have widely been used as resins for preparing heated roll-fixable toners, because of their good ability of dispersing, therein, various additives for toners such as carbon black, good ability of wetting transfer paper and excellent fixability or the like.
In addition, the quantity of paper commonly used in copying apparatuses and optical printers have rapidly been increased recently and the amount of paper abandoned or thrown into fire is correspondingly increased. This becomes a serious problem from the viewpoint of concervation of natural resources and the techniques for reclaiming and reusing such paper commonly used have been increasingly important from the viewpoint of prevention of environmental pollution and concervation of natural resources. To this end, it is desirable that the toner used in copying apparatuses and optical printers be able to efficiently be decomposed through the alkali hydrolysis during deinking and reclaiming processes for paper and efficiently be removed from the fibers of the paper. In this respect, the styrene binder resins usually used are hardly hydrolyzed with an alkali. On the other hand, it is known that resins for toner comprising degradable polyesters such as poly-( .alpha.-hydroxycarboxylic acid) exhibit high deinkability and are thus useful in the paper-recycle (see, for instance, WO 92/01245). Moreover, abandoned toners and/or resins for toners are sometimes buried in the ground. However, the conventional resins for toners are insufficient in biodegradability. For this reason, there has been desired for the improvement of these resins for toners from the viewpoint of disposition of waste and environmental pollution. In this respect, it has been known that the toners comprising degradable polyesters are decomposed in the ground (see, for instance, Japanese Un-examined Patent Publication (hereunder referred to as "J.P. KOKAI") No. Hei 4 -179967 and U.S. Pat. No. 5,004,664).
Moreover, there have also been known various methods for synthesizing poly-(.alpha.-hydroxycarboxylic acids) as resins for toners which comprise once converting an .alpha.-hydroxycarboxylic acid into a cyclic dimer thereof, then separating and purifying the cyclic dimer and subjecting the dimer to the ring opening polymerization in the presence of a catalyst such as an aluminum catalyst.
The crystalline resins for toners each has an extremely high viscosity at a temperature lower than the melting point thereof, but has an extremely low viscosity at a temperature higher than the melting point. This inevitably increases the lowest temperature (or the lowest fixing temperature) required for fusing the toner and for adhering it to paper. In addition, the toner is heated on the surface of a hot rolls, the viscosity thereof is thus substantially reduced and this results in the so-called hot-offset phenomenon and in turn to contamination of the hot roll surface. The resins for toners must start flowing at a low temperature and must have sufficiently high viscosities at a high temperature in order to simultaneously satisfy the requirements for fixability and resistance to hot-offset.
The conventional degradable polyester-based resins for toners are hardly pulverized and it is accordingly difficult to use them as binder resins for toners having a particle size on the order of 10 .mu.m, 90% of which is occupied by the binder resins. Therefore, these degradable polyesters should further be improved. Moreover, polyesters other than degradable polyesters as well as styrene polymers can easily be pulverized, but are inferior in hydrolyzability, degradability with an alkali and biodegradability and thus these polymers require further improvement.
Furthermore, products obtained by dimerizing an .alpha.-hydroxycarboxylic acid into a cyclic dimer through dehydration are in general lactides and glycollides used in the preparation of polyesters through the ring opening polymerization and, therefore, the polyesters thus prepared are too expensive to use as resins for toners. There has thus been desired for the development of cost-saving methods for preparing degradable polyesters usable as toner-resins through direct polymerization. In addition, the ring opening polymerization which is cation ring opening polymerization and accordingly requires the use of non-aqueous solvents and removal of ionic species serving as Polymerization-terminators. This leads to reduction of production efficiency. Further polyols and polycarboxylic acids generally used in the production of polyesters cannot be used or the monomers usable in the production of such polyesters are limited to only cyclic esters. In this polymerization, it is not easy to control physical properties required for resins used in toners and to perform copolymerization with a variety of monomers for controlling the balance between the degradability and physical properties of the polyester.
In addition, each resin used in toners comprising a degradable polyester has a hydroxyl group on one end and a carboxyl group on the other end and the resin limits the charging properties of the resulting toners or can provide only toners capable of being strongly positively and weakly negatively charged due to the presence of carboxyl groups having an ability of being strongly negatively charged. Accordingly, the resulting toner cannot be used in the copying apparatuses equipped with organic light-sensitive materials.
In this case, the resin can further be polycondensed to reduce the content of the terminal carboxyl groups, but this leads to an increase in the molecular weight of the resulting resin and hence to a decrease in the grindability thereof. As a result, the resin does not have a particle size suitable for the production of excellent toners. On the other hand, the forcible pulverization thereof requires extremely high pulverization energy and is impractical from the economical standpoint.